The field of the invention is that of lithography in integrated circuit processing, in particular in patterning a wafer before an etch step.
In the course of processing integrated circuits, a standard sequence is that of putting down a layer of material, then depositing a layer of photoresist, patterning the photoresist by projecting a pattern on it and developing the resist to produce a pattern of open areas that expose the material, with the other areas still covered by the resist.
The etching process generates heat, so that it is preferred to extend the pattern to the edges of the wafer, even though the particular integrated circuit at the edge will not fit on the wafer and therefore cannot be used. The reason for this is that performing the etching at the edge tends to spread out the heat produced by the etching more uniformly than if this were not done, thus reducing stress on the wafer and the possibility of distortion.
A drawback of this approach is that, while it accomplishes its primary purpose of reducing stress, it sometimes permits or encourages the development of closely spaced sets of narrow cracks in the silicon wafer material, referred to as “Black” silicon, since it absorbs light very strongly. The narrow slivers of silicon between the cracks tend to break off, producing particles that cause defects in the integrated circuit, and other problems.
Some etching tools have “shadow rings”, circular pieces of an etch-resistant material that blocks the etching process at the edge, as shown in FIG. 8 and discussed later. These tools have problems if the shadow ring contacts the wafer, which would contaminate the tool. Another drawback of using rings is that etching uniformity is degraded. In addition, the rings, being mechanical objects, are not positioned as precisely as the lithographic patterns, requiring that space be left as a buffer between the ring position and the closest chip on the wafer.
The black silicon phenomenon is a problem only very close to the edge of a wafer, about 5 mm in current technology. It would be desirable to be able to block the etching in deep trench etching steps as required with a controllable pattern that can be aligned with great precision.